1. Field of the Invention
The present invention relates generally to chromatic dispersion compensation, and particularly to polarization mode dispersion compensation for chirped fiber gratings.
2. Technical Background
One problem associated with upgrading the channel capacity of optical communication systems is compensating the dispersion of the optical signals. In order to increase the capacity of optical communication systems it is necessary to cancel the accumulated dispersion over the entire operational wavelength range. Thus, accurate control of dispersion control is required. Typically, optical communication systems will require dispersion control of less than 100
picoseconds per nanometer and potentially as low as 20 picoseconds per nanometer. Dispersion may be expressed as the variation in group delay, xcfx84, with respect to wavelength xcex or D(xcex)=dxcfx84/dxcex. Alternately, dispersion may be expressed mathematically as:
D(xcex)=D(xcexc)+Dxe2x80x2(xcexc)(xcexxe2x88x92xcexc)xe2x80x83xe2x80x83(1)
where,
D(xcexc) is the dispersion (second order); and
Dxe2x80x2(xcexc) is the dispersion slope (third order) or S.
One approach to dispersion compensation in optical communication systems is to intersperse sections of dispersion compensating optical waveguide fiber between segments of transmission of optical fiber. Some factors that influence the performance and design of dispersion compensation modules include; changing traffic patterns, variations in the optical power of the optical signals, temperature fluctuations along the length of the optical fiber waveguides, installation effects on the cable, modulation format, channel spacing and irregularities with the optical waveguide fiber. Because the magnitude of dispersion in optical communication systems may change, dispersion compensation using dispersion compensating fiber will not be optimal for every channel and a tunable dispersion compensation is required.
Dispersion may be compensated for dynamically during the operation of the optical communication system in order to minimize time-dependent effects such as, for example, the variation in the dispersion characteristics of optical waveguide fibers resulting from fluctuations in temperature.
One proposed approach for dispersion compensation uses fiber Bragg gratings as tunable dispersion compensators for a single channel. In one proposed configuration, a single non-linearly chirped fiber Bragg grating is strained to vary the dispersion within the band of wavelengths of interest. Another proposed configuration uses temperature to shift the wavelength of the entire dispersion curve. This approach to dispersion compensation introduces some amount of dispersion slope across the passband. The thermally tuned configuration dynamically alters the amount of chirp along the length of the grating. The thermally tuned configurations proposed thus far require complex tuning mechanisms to control the dispersion compensation as well as requiring a second tuning mechanism to maintain a constant grating center wavelength. Thus there is a need for a less complex, tunable dispersion compensator for optical communication systems.
Polarization mode dispersion is the maximum difference of group delay as a function of polarization for an optical component. For optical components, polarization mode dispersion is a deterministic quantity and is the difference in group delay between the two principal states of polarization. In the case of linearly chirped fiber Bragg gratings group delay is a linear function of wavelength and the polarization mode dispersion is simply related to the birefringence of the fiber in the grating region by:
PMD=|CD|xc3x97xcex94xcexBxe2x80x83xe2x80x83(2)
where
PMD is the polarization mode dispersion;
CD is the chromatic dispersion; and
xcex94xcexB is the Bragg wavelength shift.
Equation (2) may also be written as:
PMD=|CD|xc3x972xcex94nxcex9xe2x80x83xe2x80x83(3)
where
PMD is the polarization mode dispersion;
CD is the chromatic dispersion;
xcex94n is the effective group index difference; and
xcex9is the grating period.
For nonlinearly chirped fiber Bragg gratings, which have an associated nonlinear group delay as a function of wavelength, the local slope of the chromatic dispersion replaces the quantity CD in equation (2) and equation (3).
The magnitude of polarization mode dispersion in linearly chirped fiber Bragg gratings reportedly ranges between about 0.25 picoseconds and about 8 picoseconds. Polarization mode dispersion compensation requirements for optical communication systems may be less than 1 picosecond.
FIG. 1 shows one proposed approach to polarization mode dispersion compensation for a single linearly chirped fiber Bragg grating. The approach uses a 3-port optical circulator 10. An optical circulator is a non-reciprocating device that transports an optical signal from one port to the next port, only one direction (i.e. 1 to 2, or 2 to 3). They are used to separate forward and backward propagating signals.
An optical signal is received by the first port 12 of the optical circulator 10. The optical circulator 10 directs the optical signal received by the first port 12 to the optical circulator""s 10 second port 14. The optical signal propagates toward a fiber Bragg grating 16. The fiber Bragg grating 16 reflects at least a portion of the optical signal back into the second port 14. The optical circulator 10 directs the reflected optical signal to the third port 18 of the optical circulator 10. The optical signal exits the third port 18 of the optical circulator 10 and is directed through a xcex/2 waveplate 20 before entering a polarization maintaining fiber 22. The polarization maintaining fiber 22 is connected to a transmission optical waveguide fiber 24. The xcex/2 waveplate 20 in combination with the polarization maintaining fiber 22 compensate for the polarization mode dispersion in the single grating 16.
FIG. 2 shows another proposed approach to polarization mode dispersion compensation for a single linearly chirped fiber Bragg grating. This approach also uses a three port optical circulator 10. An optical signal is received by the first port 12 of the optical circulator 10. The optical circulator 10 directs the optical signal received by the first port 12 to the optical circulator""s 10 second port 14. One end of a polarization maintaining fiber 22 is optically coupled to the second port 14. The other end of the polarization maintaining fiber 22 is optically coupled to a fiber Bragg grating 16. The optical signal propagates through the polarization maintaining fiber 22 before at least a portion of the optical signal is reflected by the fiber Bragg grating 16. The reflected optical signal propagates back through the polarization maintaining fiber 22 and is introduced into the second port 14 of the optical circulator 10. The optical circulator 10 directs the reflected optical signal out of the third port 18. Typically, the third port 18 is optically coupled to an optical waveguide fiber 24.
Pulse broadening induced by polarization mode dispersion is important in chirped Bragg gratings used as chromatic dispersion compensators in optical communication systems, and has been found to be of particular importance in high speed optical communication systems.
One aspect of the invention is a chromatic dispersion compensator. The chromatic dispersion compensator includes a first optical circulator. The first optical circulator has a first optical port; a second optical port; and a third optical port. The chromatic dispersion compensator further includes a first grating coupled to the second optical port. A polarization controller is coupled to the third optical port. The chromatic dispersion compensator further includes a second optical circulator. The second optical circulator has a fourth optical port coupled to the polarization controller; a fifth optical port; and a sixth optical port. The chromatic dispersion compensator also includes a second grating coupled to the fifth optical port.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operations of the invention.